Evidence for magnetic boundary layer accretion in RU Lup

A spectrophotometric analysis^★

¹ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
² INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁴ Department of Physics & Astronomy, Stony Brook University, Stony Brook, NY, 11794-3800, USA
⁵ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁶ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ SUPA, School of Science and Engineering, University of Dundee, Nethergate, DD1 4HN, Dundee, UK
⁸ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
¹⁰ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Italy
¹¹ ASI, Italian Space Agency, via del Politecnico snc, 00133 Rome, Italy

^★★ Corresponding author; armeni@astro.uni-tuebingen.de

Received: 11 June 2024
Accepted: 9 August 2024

Abstract

Context. It is well established that classical T Tauri stars accrete material from a circumstellar disk through magnetic fields. However, the physics regulating the processes in the inner (0.1 AU) disk is still not well understood.

Aims. Our aim is to characterize the accretion process of the classical T Tauri Star RU Lup.

Methods. Optical high-resolution spectroscopic observations with CHIRON and ESPRESSO were obtained simultaneously with photometric data from AAVSO and TESS.

Results. We detected a periodic modulation in the narrow component of the He I 5876 line with a period that is compatible with the stellar rotation period, indicating the presence of a compact region on the stellar surface that we identified as the footprint of the accretion shock. We show that this region is responsible for the veiling spectrum, which is made up of a continuum component plus narrow line emission that fills in the photospheric lines. An analysis of the high-cadence TESS light curve reveals quasi-periodic oscillations on timescales shorter than the stellar rotation period, suggesting that the accretion disk in RU Lup extends inward of the corotation radius, with a truncation radius at ~2 R_*. This is compatible with predictions from three-dimensional magnetohydrodynamic models of accretion through a magnetic boundary layer (MBL). In this scenario, the photometric variability of RU Lup is produced by a nonsta-tionary hot spot on the stellar surface that rotates with the Keplerian period at the truncation radius. We also qualitatively discuss how more complex hot spot shapes may generate the same variability pattern. The analysis of the broad components of selected emission lines reveals the existence of a non-axisymmetric, temperature-stratified flow around the star, in which the gas leaves the accretion disk at the truncation radius and accretes onto the star channeled by the magnetic field lines. The unusually rich metallic emission line spectrum of RU Lup might be characteristic of the MBL regime of accretion.

Conclusions. Our extensive multiwavelength database of RU Lup reveals many similarities to predictions from the scenario of accretion through a magnetic boundary layer. Alternative explanations would require the existence of a hot spot with a complex shape, perhaps made of two brighter knots, or a warped structure in the inner disk.